The Galactic Republic had existed for more than 25,000 years before the events that led to the Separatists, Empire, Rebellion and New Republic.

The Republic (often referred to as the Old Republic) had become stagnant and corrupt over many millenia of rule and the Sith had a long-term plan to infiltrate the ruling elite and bring down the Republic so they could form an empire under Sith dictatorship.

The Galactic Senate had representatives from a huge number of planets and factions and until Palpatine began manipulating events, most people had a voice.

The Republic was replaced by the Empire and the Senate dissolved at the time of the Battle of Yavin (BBY) and 5 years later the New Republic was declared.